1. Field of the Invention
The present disclosure relates to an intercom system that includes a front door slave unit having an attached camera and a master unit, and a communication method of the intercom system.
2. Description of the Related Art
An intercom system which performs a telephone conversation between a front door slave unit and a master unit has been widespread used. The front door slave unit is disposed at the front door and the master unit is disposed in the room. Recently, a change of such an intercom system to a so-called television intercom system attracts attention. In the television intercom system, an image of the front door can be displayed in the master unit.
In the conventional intercom system that does not transmit an image signal, analog transmission using a two-wire cable is employed in many cases. When the intercom system is changed, if using of the two-wire cable which has been already provided is possible, high advantages in terms of time or cost are obtained. In a case of the television intercom system, if an increase of the quantity of transmitted data due to an image signal is considered, digital transmission is more appropriate than the analog transmission.
For example, an intercom system that performs digital transmission for a signal by using the two-wire cable is disclosed in Japanese Patent Unexamined Publication No. 2007-124227.
In the technology (hereinafter, “the related art”) disclosed in Japanese Patent Unexamined Publication No. 2007-124227, a front door slave unit performs digital modulation in a state where a control signal, an image signal, and a sound signal of the front door slave unit are packetized, and the front door slave unit transmits a result of digital modulation to the master unit. The master unit performs digital modulation in a state where a control signal and a sound signal of the master unit are packetized, and the master unit transmits a result of digital modulation to the front door slave unit. At this time, the front door slave unit and the master unit perform digital modulation by using different frequency bands, and thus full duplex communication is realized.
According to such a related art, it is possible to perform a change to the television intercom system by using the two-wire cable which has been already provided.